This invention relates to phase-lock-loops (PLL) and, in particular, to a circuit for providing a signal to indicate when a phase-lock-loop circuit is in lock.
Phase lock loop circuits are utilized in many applications to provide an output signal that is of the same frequency and phase of an input reference signal. The main components of a PLL circuit comprise a phase/frequency detector, a loop filter, and a voltage controlled oscillator (VCO), as is well known in the art. When in lock, the output signal of the VCO is substantially the same frequency and phase as the input reference signal which is typically applied to an input of the phase/frequency detector. The phase/frequency detector typically has two inputs and two outputs. The two inputs include the input reference signal and the VCO feedback signal while the two outputs provide an UP and a DOWN signal. The phase/frequency detector typically compares the input reference signal to the VCO feedback signal and operates such that if the frequency of the VCO signal is lower than that frequency of the input reference signal, the UP signal of the phase frequency detector goes low thereby indicating that the operating frequency of the VCO must be increased. Furthermore, if the frequency of the VCO signal is at a higher frequency than the input reference signal, the DOWN signal goes low indicating that the operating frequency of the VCO must be decreased. If both the UP and the DOWN signals are at a logic high, then the input reference signal and the VCO feedback signal are substantially equal in frequency and phase and, thus, the phase-lock-loop circuit is in lock.
As the phase-lock-loop circuit is achieving lock on an input reference signal, the VCO feedback signal will overshoot and undershoot the input reference signal before a steady state frequency lock condition is achieved. Therefore, for these temporary frequency lock conditions, which can typically last for several cycles, the UP and the DOWN signals will be a logic high thereby falsely indicating that the phase-lock-loop circuit is in lock.
Further, there typically exists other circuitry that is responsive and becomes activated when the phase-lock-loop circuit of the system is in lock. Therefore, in many applications, it may be essential to knowing when the phase-lock-loop circuit is in lock with a reference signal.
Hence, a need exists for a circuit to indicate when a phase-lock-loop circuit is in lock.